This disclosure relates generally to field devices for use in industrial process networks. More particularly, the present disclosure relates to message processing and data transfer using communications units of a field device.
In a typical industrial plant, a distributed control system (DCS) is used to control many of the industrial processes performed at the plant. Typically, the plant has a centralized control room having a computer system with user input/output (I/O), disc I/O, and other peripherals. Coupled to the computing system are a controller and a process I/O subsystem.
The process I/O subsystem includes I/O ports which are connected to various field devices throughout the plant. Field devices include various types of analytical equipment, silicon pressure sensors, capacitive pressure sensors, resistive temperature detectors, thermocouples, strain gauges, limit switches, on/off switches, flow transmitters, pressure transmitters, capacitance level switches, weigh scales, transducers, valve positioners, valve controllers, actuators, solenoids, and indicator lights. The term “field device” encompasses these devices, as well as any other device that performs a function in a distributed control system.
Traditionally, analog field devices have been connected to the control room by two-wire twisted pair current loops, with each device connected to the control room by a single two-wire twisted pair. Analog field devices are capable of responding to or transmitting an electrical signal within a specified range. In a typical configuration, it is common to have a voltage differential of approximately 20-25 volts between the two wires of the pair and a current of 4-20 milliamps running through the loop. An analog field device that transmits a signal to the control room modulates the current running through the current loop, with the current proportional to the sensed process variable. On the other hand, an analog field device that performs an action under control of the control room is controlled by the magnitude of the current through the loop, which is modulated by the I/O port of the process I/O system, which in turn is controlled by the controller. Traditional two-wire analog devices having active electronics can also receive up to 40 milliwatts of power from the loop. Analog field devices requiring more power are typically connected to the control room using four wires, with two of the wires delivering power to the device. Such devices are known in the art as four-wire devices and are not power limited, as are two-wire devices.
Historically, most traditional field devices have had either a single input or a single output that was directly related to the primary function performed by the field device. For example, the only function implemented by a traditional analog resistive temperature sensor is to transmit a temperature by modulating the current flowing through the two-wire twisted pair, while the only function implemented by a traditional analog valve positioner is to position a valve between an open and closed position, inclusive, based on the magnitude of the current flowing through the two-wire twisted pair.
More recently, hybrid systems that superimpose digital data on the current loop have been used in distributed control systems. One hybrid system is known in the control art as the Highway Addressable Remote Transducer (HART) and is similar to the Bell 202 modem specification. The HART system uses a half-duplex master-slave protocol. Typically, the master sends a command and expects a reply. In a complementary manner, the slave typically waits for a command and sends a reply in response. Each command or reply can be considered a message, varying in length from a few bytes to as many as 269 bytes. The message consists of asynchronous serial data transmitted at 1,200 bits per second (BPS). Transmission is accomplished through frequency-shift-keying (FSK), such that a logical 1 is represented by a 1,200 Hertz (Hz) signal and a logical 0 is represented by a 2,200 Hz signal. These HART protocol signals are modulated onto the two-wire communications line carrying the D.C. power.
Using the HART communication protocol, a message transaction is initiated by the central or control station sending a command to a specified field device, usually by addressing the field device with a unique address. The command may, for example, direct the field device to reply with information as to its status, such as current pressure sensed, or other status information. Upon receipt of the command from the control station, the field device sends a reply to the control station which is received and processed. The HART system permits the control station to conduct approximately two or three transactions per second. Accordingly, the HART protocol, while enabling such digital messages to be transmitted over the analog medium, can be relatively slow to send and receive such messages. The data rate of incoming and outgoing HART messages can require the processor to service such messages in intervals to avoid excessive delays of other high priority tasks of the processor. This, in turn, can require multiple interrupts of the processor per message, thereby increasing task-switching and other loads on the processor and increasing the complexity of implementing the communications protocol on the processor.